Some measurement instruments require calibration for accurately measuring the electrical response of a device under test (DUT) to test signals transmitted by the measurement instrument. For example, measurement calibration of a vector network analyzer (VNA) used to measure scattering (S-) parameters for a DUT is necessary as part of the setup to measuring the DUT. Measurements performed without calibration will be limited due to imperfections.
A VNA is a reflectometer-based electronic instrument that can be used to measure the frequency response (magnitude and phase) of a DUT such as an electrical network, component, circuit, or sub-assembly. A VNA makes use of a frequency sweeping source or stimulus, directional couplers, and one or more receivers that provide ratioed amplitude and phase information such as reflection and transmission coefficients. VNAs commonly measure s-parameters because reflection and transmission of electrical networks are relatively easy to measure at high frequencies. VNAs are often used to characterize two-port networks such as amplifiers and filters, but they can be used on networks with an arbitrary number of ports.
The basic architecture of a network analyzer involves a signal generator, a test set, one or more receivers and display. In some setups, these units are distinct instruments. Most VNAs have two test ports, permitting measurement of four S-parameters (S11, S21, S12 and S22), but instruments with more than two ports are available commercially.
A VNA achieves highly accurate measurements by correcting for the systematic errors in the instrument and the characteristics of cables, adapters and test fixtures. The process of error correction is commonly just called calibration and may be performed by an engineer several times in an hour. Sometimes it is called user-calibration to indicate the difference from periodic calibration by a manufacturer.
In a typical 1-port reflection calibration, the user measures three known standards, usually an open (O), a short (S) and a known load (L). From these three measurements the network analyzer can account for directivity errors resulting from the portion of the source signal that never reaches the DUT, source match errors resulting from multiple internal reflections between the source and the DUT, and reflection tracking errors resulting from all frequency dependence of test leads, connections, etc.
A more complex calibration is a full 2-port reflectivity and transmission calibration. For two ports there are 12 possible systematic errors analogous to the three above. The most common method for correcting for these involves measuring a short, load and open standard on each of the two ports, as well as transmission between the two ports.